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The eccentricity of Saturn's orbit around the Sun could be responsible for the unusually uneven distribution of lakes over the polar regions of Saturn's largest moon Titan.

Saturn's orbit around the Sun is not perfectly circular, meaning that different parts of Titan are exposed to different amounts of sunlight throughout its year. A Saturnian year is equivalent to 29.5 Earth years, and every 15 years on Titan the seasons at the poles reverse.

The northern and southern hemispheres of Titan, showing the abundance of lakes in the north compared with southern latitudes. The new hypothesis suggests a long-term, recent flux of methane from south to north. Image: NASA/JPL/Caltech/UA/SSI.

The deviations of Earth's orbit from a perfect circle are known to drive long-term ice age cycles on our planet. Known as Milankovitch cycles, they are linked to changes in solar radiation, which affect the global redistribution of water in the form of glaciers. Scientists propose that a similar effect may be occurring in the outer Solar System, too.

Cassini observations have already revealed that liquid methane and ethane lakes in Titan's northern high latitudes cover 20 times more area than lakes in the southern high latitudes, with more partially filled and empty lakes in the north. Two previously contending ideas for this observation were based on differences in topography that would influence the way in which liquid drains into the ground, and seasonal variations that would preferentially fill northern lakes while drying out southern lakes.

However, says Oded Aharonson, associate professor of planetary science at Caltech and lead author of the new Nature Geoscience paper, seasonal variations aren't enough to account for the significant differences – the north polar region has roughly three times as many dried-up lake basins as the south and seven times as many partially filled ones.

“The seasonal mechanism may be responsible for part of the global transport of liquid methane, but it’s not the whole story,” he says.

A Cassini radar image of what is the largest body of liquid ever found on Titan's north pole (left), compared to Earth's Lake Superior (right). This feature on Titan is at least 100,000 square kilometres, while Lake Superior occupies 82,000 square kilometres. Image: NASA/JPL/GSFC.

This is where Saturn's journey around the Sun may hold the answer. “We propose that, in this orbital configuration, the difference between evaporation and precipitation is not equal in opposite seasons, which means there is a net transport of methane from south to north,” says Aharonson. This imbalance would lead to an accumulation of methane, and hence the formation of many more lakes, in the northern hemisphere.

But this situation only applies to the present epoch. Over the time scales of tens of thousands of years, Saturn’s orbital parameters vary, at times causing the moon to be closer to the Sun during its northern summer and farther away in southern summers, therefore producing a reverse in the net transport of methane that results in an abundance of lakes in the southern hemisphere. Aharonson and colleagues say that this situation arose 32 thousand years ago.

“Like Earth, Titan has tens-of-thousands-of-year variations in climate driven by orbital motions,” says Aharonson. “On Titan, there are long-term climate cycles in the global movement of methane that make lakes and carve lake basins. In both cases we find a record of the process embedded in the geology. We may have found an example of long-term climate change, analogous to Milankovitch climate cycles on Earth, on another object in the Solar System.”

Cassini has been exploring the Saturnian system since 2004.

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